Recombinant DNA techniques were employed to construct a defined system to study the control of integration of phage lambda: 1) pBR322 plasmids carrying the Pint promoter and int gene or Pint promoter and the galK gene were constructed. 2) lambda CII gene was fused to its pLN operon carried in pBR322 plasmid. 3) The Pint plasmid (TetR) and the pLNCII plasmid (AmpR) were forced to co-exist in E. coli galK- lysogens with the use of drugs. We find that a) lambda CII gene product is sufficient to turn on Pint promoter in the absence of cIII gene product in vivo and b) in a coupled transcription-translation system in vitro, CII allows the synthesis of Integrase from Pint-int DNA, or galactokinase from Pint-galK pllsmid DNA. The CII protein is being purified and its mode of control on lambda integrase synthesis is under study. New plasmid cloning vehicles containing unique SacI, HpaI and BglII restriction sites have been constructed.